The invention relates to an improved brake for motor vehicles and more particularly to an improved brake for the trailers of such vehicles.
It has been usual for many years to use so-called internal expanding shoe brakes for motor vehicles. Such brakes comprise a brake drum secured to the wheel hub and nested as close as possible into the wheel rim so as to obtain a large brake drum diameter. The brake shoes are situated in a so-called brake plate which is arranged on the body of the axle, are hydraulically or pneumatically operated and are arranged in a multiple arrangement and with complicated bearing means. These internal expanding shoe brakes, however, have many disadvantages and at the present time, they are only just able to meet the legal requirements regarding continuous braking for heavy axles such as are used on trailers.
It has been attempted to increase the braking capacity of internal expanding shoe brakes with trailer axles by arranging additional internal expanding shoe brakes towards the center of the axle. But since these additional brakes have not proved to be very successful, and since they involve a considerable capital outlay, this arrangement has not been widely adopted. In order to meet the legal requirements regarding continuous braking, the brake drums have been strengthened, widened and increased in diameter. However the disadvantages still exist.
Within the brake drum, the brake shoes are mounted on the body of the axle and are so arranged that they come to abut the brake drum by opening out of the shoe ends. Owing to the direction of rotation of the brake drum, one shoe runs against its bearing; whereas, the other runs off the bearing. The brake linings are, thus, subjected to very different amounts of wear. The running-on shoes develop a higher application pressure which results in considerable stress on the brake lining and brake drum. As a result, both the lining and brake drum become heated to too great an extent, and are subjected to premature wear. Moreover it is difficult to dissipate the heat produced in the lining and brake drum. Also, there is often an undesirable "grabbing" effect which extends to the extent of resonance in the shoes and drum and causes a high-frequency whistling that increases particularly at night so as to be a very unpleasant noise.
The aforesaid stress occurs more particularly on trailer brakes. These brakes also have to deal with the additional continuous braking operations which, in trucks, are caused by the engine brake effect resulting from throttling the exhaust piping.
The trailer brakes must also be adjusted with a certain amount of lead time so that the cab-trailer train remains extended when a braking operation is carried out and so that jack-knifing is prevented. This measure also subjects the trailer brake to additional stress during each braking operation. In addition, dirt collects in the brake drum, and in a braking operation, results in much wear on the brake drum.
The excessive stress on the brake lining and brake drum requires excessively frequent renewal of these parts, which is expensive and time consuming, and causes many failures. During continuous braking, there is a drop in braking power because the drum and brake linings become hot. This drop in braking power is particularly disadvantageous and dangerous when carrying out a full braking operation with a hot brake.
The known and relatively effective disc brakes are, more and more, taking the place of the internal expanding shoe brakes, particularly in private motor cars. Disc brakes have not been widely adopted, however, in the case of heavy axles. The arrangement and operation of the brake shoes in the housing, particularly with respect to having the shoes engage over the external periphery of the disc brake, is difficult owing to the space conditions, especially if high braking forces have to be transmitted.
The electrical and hydraulic so-called retarders which are also known and which are used additionally to affect continuous braking operations with the friction brake have also not been widely adopted. The outlay with regard to cost, weight and space occupied has just been too great.
Therefore, the present invention has as its object to provide an improved brake which fully satisfies the legal requirements regarding continuous braking, which is simple in construction, which allows easy access, which allows facile replacement of parts, which can transmit considerable braking forces, and which minimizes the disadvantageous effects on other parts of the brake resulting from the friction heat produced. According to the present invention, this object is achieved providing an improved brake wherein a brake ring is connected securely to the wheel hub. At least two brake shoes act on the external peripheral surface of the brake ring, and each are arranged at one end of a two-arm lever. The other ends of the two-arm levers are acted upon by a hydraulic operating device.
The hydraulic operating device may be a hydraulic or pneumatic cylinder which is connected to the other end of one of the two-arm levers and whose piston rod is connected to the other end of the other two-arm lever. However, it is also possible to provide a separate hydraulic operating cylinder for each two-arm lever. In such a situation, the cylinders would preferably bear against the body of the axle and their piston rods would engage the other ends of the two-arm levers. Furthermore, the hydraulic cylinder can have a floating-mounting and can be situated outside the axle body or can extend through the axle body or can be situated at one side of the axle body. In the latter case a rod can be used to connect the cylinder to the two-arm lever situated at the other side of the axle body.
The pivot points of the two two-arm levers are formed in bearing brackets on the axle body. The levers can extend in a rectilinear manner if two hydraulic cylinders, which are supported at the center of the axle, are used or if one hydraulic cylinder is used and its piston rod has an extension extending through the body of the axle. In the case of cylinders mounted outside the axle body, elbowed levers are to be used.
Mounted on the one end of each of the two-arm levers is a brake shoe which is connected pivotably and positively to the two-arm lever so that a uniform wear on the brake lining is ensured. More particularly, by arranging the hydraulic operating device in a so-called floating mounting or arrangement between the appropriate other ends of the two-arm levers, brake balance is ensured. The hydraulic operating device is preferably actuated by means of compressed air of the vehicle by means of a hydraulic transmitter cylinder that supplies the hydraulic operating cylinder which is located directly adjacent to the levers. By this arrangement, there is no need for special re-adjustment of the improved brake when there has been wear on the brake linings since the travel of the operating cylinder which is available is sufficient for re-adjustment.
The brake ring, made from appropriate material and with appropriate dimensions, is secured to the wheel hub 13 by means of a spacer ring. The axis of the brake ring coincides with the axes of the stub axle. The spacer element can be constructed as a bladed wheel so that on rotation, it can be utilized to draw cooling air through the brake ring and blow it out again outside the brake ring and between the ring and the adjacent rim of the wheel. The brake ring can be connected with the spacer ring by means of bolts situated in blind holes in the brake ring or the brake ring and the spacer ring can form a single unit. In the latter case, the combined spacer ring-brake ring would be secured to the hub. In the first case, the hub is connected securely to the spacer ring. But it is also possible to construct all the parts independently and to connect them by screws or other suitable detachable securing means in such a manner that the spacer ring is connected detachably to the hub and the brake ring, itself, is connected detachably to the spacer ring.
The improved brake, according to the present invention, provides a brake which has brake linings of long service life, which can tolerate high application pressures on brake linings and brake ring and which can tolerate higher temperatures than in the case of drum brakes, and even disc brakes, if in the case of the latter the hydraulic operating cylinder is situated directly in the zone radiating heat. Also, the disadvantage of brake drum wear, which is so significant in the case of drum brakes, does not occur. Likwise the improved brake of the present invention minimizes the difficulty of replacing brake linings and the differing amounts of wear on the brake linings by running-on and running-off shoes.
Similarly, there is no drop in braking power during continuous braking operations. Above all, the drop in braking power, when carrying out full braking with the brake hot, is obviated. In order to avoid the excessively high pressure between the shoes and the brake ring that might build up in a continuous braking operation as a result of the diameter of the brake ring expanding due to the heat of friction, the electro-pneumatic control valve of the continuous brake is preferably preceded by a conventional interval switch. As a result of an interruption in current at suitable intervals, the interval switch thus controls the operation of the control valve so that a brief pressure drop is produced at suitable intervals. These brief pressure drops obviate a continual rise in application pressure on the part of the brake shoes against the brake ring.
Accumulations of dirt at the shoe brakes within the brake drum, which can cause premature wearing of the lining and drum, cannot occur with a ring brake of the present invention. On the contrary, in the improved brake of the present invention, dust and dirt which arrive are stripped off the brake ring and drop out.
The invention will be explained in detail in connection with the preferred embodiment of the present invention shown in the drawings.